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developedon the cam frame is transfered by the operating
link to the backspace bellcrank. The bellcrank rotates and
pulls the backspace pawl link to the right. The backspace
pawl is mounted on a diagonally cut elongated hole; as it is
pulled to the right, it is cammed into the escapement rack
(Fig. 66-A). Two springs, attached to the head of the back-
space pawl, cause the head to delay any movement to the right
unti I the backspace pawl is moved into engagement with the
escapement rack (Fig. 66-8). When the tooth of the back-
space pawl fully engages the teeth of the escapement rack, the
continuing pull of the linkage starts to move the carriage to
the right (Fig. 66-C). The motion continues until the car-
riage moves for enough to allow the escapement pawl to snap
into the next tooth of the escapement rack (Fig. 66-D). At
this point the motion of the backspace pawl is stopped by the
backspace pawl stop and it,in turn, halts any further motion
of the carriage to the right. The backspace pawl has now
completed two jobs. First, it overcame carriage tension and
mass when it moved the carriage one space to the right.
Second, it locked the carriage in the new position long enough
for the carriage momentum to the right to be blocked by the
pawl and its stop. The backspace cam now drops off its high
point and all parts return to rest.
A
~
Backspace Pawl At Rest
Against Guide lug
B
CD
Backspace Pawl Slides On Diagonally
Cut Mountingllole To Engage Rack
Escapement Rack
fll>==-==-Sto p
Figure 66. Backspace Pawl Sequence Of Operation
32
Intarloct I r "
---!>
'-Backspace Pawl Clears Rack
'-= ....
f-r.....,.~-.lJ'
And Stop When Backspace Is
Backspace I nterlock Operated
Operated With Interlock
Actuated
Figure 67. Backspace Interlock
Backspace Carriage Return Interlock
Figure 68. Early Backspace Interlock.
A backspace interlock is also mounted on the backspace pawl
bracket. The interlock is actuated any time the pawl release
lever is operated by tab, carriage return or hand carriage
release. The interlock does not prevent the backspace pawl
from rotafi.ng, but prevents its entry into the escapement rack
and also prevents the pawl release lever from removing the
escapement pawl from the rack any time the backspace pawl
is in the escapement rack (Fig. 67).
Another interlock, called the backspace-carriage return
inte~
lock, was used on earlier machines t.o prevent the clutch from
latching during simultaneous operation of backspace and car-
riage return (Fig. 68). It did this by rotating as the backspace
pawl entered the rack; one end of it following the backspace
pawl, the other end contacted the carriage return-tab inter-
lock. This prevented the carriage return-tab interlock from
rotqting if the carriage return were operated, and consequent-
ly prevented the clutch latch from engaging the clutch lever.
The spring on the backspace-carriage return interlock had to
be of gre<:lter strength than the clutch latch spring for success-
ful operation.
Current Model 81 machines use the extension of the carriage
return-tab interlock, that extends behind the backspace inter-
lock, for the same purpose. If the backspace pawl is in the
escapement rack, the backspace interlock cannot be rotated
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